1. Field of the Invention
The present invention relates to an ink-jet recording ink and an image forming method, specifically, an ink-jet recording ink comprising a pigment as a coloring component, and an image forming method using the ink-jet recording ink.
2. Description of the Related Art
The ink-jet recording method has been spreading and developing rapidly due to low costs for materials, high-speed recording, little noise at the time of the recording, and easy color recording provided by the method.
The ink-jet recording method includes the method of applying pressure to ink by means of a piezoelectric element to discharge droplets, the method of generating bubbles in ink by heating to discharge droplets, the method of using ultrasonic waves, and the method of absorbing and discharging droplets by electrostatic force. Various inks are used correspondingly to these methods.
As inks for ink-jet recording, water-based ink, oil-based ink, and solid (melting-type) ink are used. As coloring matters used in these inks, dyes and pigments are known. Output image formed with the ink comprising a dye has insufficient fastness, that is, storage stability of image after image formation is insufficient. Thus, the use of pigment ink, which generally has superior fastness and is hard to decolorize, has been investigated.
Pigment ink is usually produced by dispersing a water-insoluble organic pigment into an aqueous medium. The production process thereof generally comprises the step of adding the pigment to the aqueous medium containing surfactant and dispersing polymer and pulverizing the pigment with hard beads by a disperser such as a sand mill, a ball mill or the like. In this case, pigment particles having large particle sizes provide ink having unfavorable storage stability and discharging property. Accordingly, it has been a big problem to establish technique for obtaining a fine and stable pigment dispersion.
For solving such a problem, a method of obtaining an organic pigment particle dispersion with a particle size of 100 nm or smaller by means of a high-speed mill dispersion using dispersing media having a particle size of 100 μm or smaller (such as polymer beads having a size of 50 μm) has been reported. (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 9-176543). In fact, a fine dispersed product can be obtained according to this method. However, according to the method, a large energy is consumed in the dispersion, the step of separating the dispersing liquid from the beads is necessary, and the process is complicated.
Another method for solving the problem comprises dissolving monomer molecules which constitute a pigment into a solution, and decreasing the solubility of the monomer molecules to precipitate pigment particles. This acid paste method, which is known as a method for producing pigment, comprises dissolving an organic pigment in concentrated sulfuric acid and adding this solution to water while the water is vigorously stirred to precipitate fine pigment particles. Another method for solving the problem is reported in which crude quinacridon is dissolved in dimethyl sulfoxide in the presence of caustic alkali and water and water is further added thereto to yield pigment particles (see, for example, JP-A No. 54-130620). In these methods, however, it is indispensable to remove the solvent (concentrated sulfuric acid and organic solvent) after the precipitation of the pigment particles. Thus, the methods are complicated as well.
There is also a method of dissolving a hydrophobic dye into a supercritical fluid and decreasing the solubility thereof to precipitate the hydrophobic dye, thereby producing a water-based ink (see, for example, JP-A No. 2001-172532). About dye-based ink, a technique of forming a clear image without white spots and dot irregularities, is also described (see, for example, JP-A No. 10-95942).
In all the above-mentioned methods, however, the finely-dispersed particles of some kinds of pigments aggregate immediately. Even the particles of pigments which are relatively stable aggregate little by little. Various kinds of dispersion stabilizers for preventing the aggregation of pigment particles are also suggested. However, stable dispersing quality has not yet been obtained.
In the case where ink containing pigment is used as ink-jet recording ink, there arises a problem that the ink remaining in nozzles in a printer precipitates so as to cause ink-clogging. Even if the nozzle wherein the ink-clogging is caused in this way is washed, it is not easy to remove the pigment which has precipitated and hardened. The ink-clogging is more easily caused as the pigment aggregates.